<< Cadillac sells an additive for their all aluminum engines.........it must be important as I remember that your warranty was voided if it was not added at change time...Ask Mr. GoodWrench for his 2 cents ! >>
When Joan got her well used Caddy, it suffered from several small coolant leaks. The additive that the owners manual insists upon was an instant cure. It is a set of six dirt-like cylinders about an inch in diameter and an inch long. I believe the stuff acts to increase the density of the coolant.
Don't know what it is, but it was a miracle to the DeVille.
The cost is about three bucks.